Studies on the structure of tubulin in microtubules, microtubule precursor sheets and extended sheets induced in the presence of zinc will be continued to characterize these arrays more fully. Work already conducted on negatively stained microtubules and precursor sheets has permitted a resolution of two classes of morphological units believed to be related to the alpha and beta polypeptide chains of tubulin. The next stage of the research will be concerned with determining the way in which these two types of subunits are arranged in microtubules with various numbers of protofilaments. Although microtubules observed in cells by electron microscopy of embedded materials reveal microtubules with 13 protofilaments, when microtubules are assembled in vitro structures with a range of protofilaments from 11-17 are observed. We will now determine the way in which the alpha and beta units are incorporated into the helical lattices of these structures. In addition, studies will be carried out to determine the extent to which the resolution achieved on negatively stained microtubules can also be achieved for microtubules in embedded and sectioned samples. The final aspect of the proposed work is to refine the structure of tubulin determined from the zinc-induced sheets by the introduction of computer techniques to correct for imperfections in the two-dimensional crystalline lattices of these arrays.